The basic premise of the anime Re:Creators is that when someone writes a book or scriptwrites a video game or something, they create an actual world where the events of the book/game/etc actually happen. Then, at the beginning of the show, some of the characters from those created worlds get transported into the "show world" (the world in which the main character [Souta] lives and in which the created worlds are works of fiction) by a Plot Device (the princess).
All of the created worlds are works of fiction that exist in the show world, but do not exist in the "real world" (the world in which you and I live), like Avalken of Reminiscence and Vogelchevalier and Magical Slayer Mamika.
I have noticed talk of a rumor (see e.g. here) that the created worlds were originally planned to be works of fiction that do exist in the real world, but that this plan fell through for one reason or another.
What is the source of this rumor, and how veracious is it? It sounds very truthy, almost to the point that I have to wonder if the source is some random guy who thought it would be cool if it actually played out that way and so originated the rumor.